The stats on the shield are random, so it may or may not be good. It is not a quest item, and you will not need it later. The dragon shield from the castle is potentially the best shield in the game, so missing this one may not be a big deal (if you missed the dragon shield you can get back in to get it).
Probably the best merchants to check for spell books would be Kistandalius or (less often) Blake in Ars Magicana and Corinna or (less often) Mpenzak in Verdistis.
In a little more detail than killerzzz described, if you quick save before running your mouse over an item/chest/merchant or hitting the alt key, you can identify the item or trade with the merchant, then quick load to get different stats or inventory. This can be done repeatedly to get the best stats or specific bonuses on an item (save after finding good candidates) or looking for specific items (such as spell books or charms) from merchants. And he's right, don't waste too much time with this; it really isn't necessary to get the best possible equipment, and if you are playing a warrior they tend to have spare skill points, so you should easily be able to put a point into Telekinesis yourself (save first, until you make sure you can get the shield).